Grinch,
Have you spent more time with the Etys/RKV combo? I'm another new RKV owner, and I've been switching between the Melos and RKV to decide which I like better.
With the Etys, the Melos has a bite that's missing from the RKV. Vocals on the RKV are more laid back, a little less intimate. Overall, this makes the RKV less involving.
When I switch to the HD600s, things even up, or perhaps the RKV comes out a bit ahead. So either the HD600 is a better match for the RKV, or the Ety's increased detail makes the difference between the amps more obvious.
Have you noticed anything similar?
I'm wondering if the issue is impedance; perhaps the 4s is too close to the 100ohm limit. I tried using my Etys without the 4p->4s adapter, figuring the differences between the amps would increase if this was case, but my listening tests were inconclusive.